Electric enamel furnace



y 1951 R. E. SKINNER ETAL ELECTRIC ENAMEL FURNACE 4 Sheets-Sheet lOriginal Filed March 15, 1949 INVENTOR$ P085327 8'. Skid/IVER By GLENNany? WEE A I w ATTOENEV.

y 10, 1951 R. E. SKINNER ET AL 2,559,683

' ELECTRIC ENAMEL FURNACE Original Filed March 15, 1949 4 Sheets-Sheet 2mmvrom 202cm E5K/1V/VEE, By GLEN/V mmmrma y 10, 1951 R. E. SKINNER ET AL2,559,683

ELECTRIC ENAMEL FURNACE Original Filed March 15, 1949 4 Sheets-Sheet 3INVENTORS ROBERT E. SKIN/YER,

BY GLENN H. Ml V TYRE muwm OTTO ENE Y6 y 1951 R. E. SKINNER ETALELECTRIC ENAMEL FURNACE 4 Sheets-Sheet 4 Original Filed March 15, 1949Patented July 10, 1951 ELECTRIC ENAMEL FURNACE Robert E. Skinner,Lakewood, and Glenn H. McIntyre, Cleveland Heights, Ohio, assignors toFerro Enamel Corporation, Cleveland, Ohio,

a corporation of Ohio Original application March 15, 1949, Serial No.81,478. Divided and this application May 4, 1949, Serial No. 91,394

1 Claim. 1

This application is a division of our co-pending application Serial No.81,478 filed March 15, 1949.

This invention relating as indicated, to method and apparatus-for makingvitreous enamel, has specific reference to method and apparatus forcontinuously electrically melting such enamels.

As is well known to those familiar with the art of glass or enamelmanufacture, vitreous enamel is a complex vitreous glass made opaque bythe addition of opacifying ingredients. The formulation of vitreousenamels are complex because of necessity. Among the features that mustbe considered in the manufacture of such an enamel are, a vitreousenamel must be fusible on a metal surface, have a high degree ofopacity, the proper coefficient of expansion and workability.

Among the ingredients employed in the manufacture of vitreous enamelsthe following usually occur: feldspar, quartz, borax, soda ash, zincoxide, sodium nitrate, bone ash, fluorspar, cryolite, sodium silicofluoride, zirconium oxide, and titanium oxide. As is very apparent whena pile of material containing the above ingredients is subjected to ahgh temperature, some such materials namely borax, soda ash, sodiumnitrate, and others will flux out at low temperatures before the otheringredients are melted. Consequently some of the materials, willseparate and flow away from the massflefore others, resulting ininconsistency in the quality of the frit produced.

Other difliculties encountered in the conventional methods ofmanufacture of vitreous enamel have been dusting loss on feeding and theloss of expensive volatile ingredients, particularly the fluorides. Asthe raw materials become heated in a fuel fired smelter the fluoridesbegin to volatilize before the rest of the materials become molten. Thusthe beneficial properties of the fluorides are in part lost before theyare able to react with the other materials in the formation of avitreous enamel.

The method or methods heretofore employed for melting the materials usedin the manufacture of vitreous enamels has been to dump a considerablequantity onto a sloping floor of a ,-fuel fired smelter and to then meltdown such mass and to permit the enamel as it melts, to run into afining chamber or to immediately discharge the same into a water bath.

Fuel fired smelters because of necessity areso constructed that the rawmaterial in the melting chamber is heated virtually only on the surfaceand depends on infrared radiation to heat the material throughout itsentire mass.

It is therefore a principal object of our invention to provide a methodof and apparatus for manufacture of vitreous enamel or the like whichwill result in a product of consistent quality.

It is a further object of our invention to provide a method of andapparatus for the manufacture of vitreous enamel whereby theloss offluorine during manufacture is at an absolute minimum.

Other objects of the invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawing setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principle of the invention may beemployed.

We have discovered a new method and apparatus for the smelting ofporcelain enamel. The apparatus operates electrically and takes severalforms.

In said annexed drawings:

Fig. 1 is a horizontal sectional view through a smelter constructed inaccordance with the principle of our invention and adapted to carry outthe process comprising our invention;

Fig. 2 is a transverse sectional view of the apparatus illustrated inFig. 1 taken on a plane substantially indicated by the line'2-2;

Fig. 3 is a transverse sectional view of the electrode controllingapparatus;

Fig. 4 is a horizontal sectional view of a second form of smelterconstructed in accordance with the principle of our invention andadapted to carry out the process comprising our invention;

Fig. 5 is a transverse sectional view of the apparatus illustrated inFig. 4 taken on a plane substantially indicated by the line 3-3 Fig. 6is a horizontal sectional view through a third form constructed inaccordance with the principle of our invention and adapted to carry outthe process comprising our invention;

Fig. 7 is a transverse sectional view of the apparatus illustrated inFig. 6 taken on a plane substantially indicated by the line 4-4.

Referring now more specifically to the drawings and more especially toFigs. 1 and 2, the smelter here illustrated in horizontal cross sectionand transverse section respectively as one embodiment of the apparatuscomprising my invention may be round or square in form and constructedof refractory materials. The hearth is built in the conventional mannerwith firebrick for the body of the smelter I and a suitable super duty"refractory for the curbwall 2 and bottom for better resistance to attackby the molten glass. The body of the smelter above the bath line, orcurbwall, need not be as thick or of as high quality refractory as infuel smelters due to absence of hot combustion gases. Also due to theabsence of hot combustion gases the volume of smelter above the bathline is at a minimum.

A projection 3 is provided in the smaller body to permit a bridge wall 4to be suspended. An overflow spout 5 is located beyond the bridge wall,the elevation of the spout determining the bath line 6. The bridge wall4 is suspended in such a way as to prevent unmelted material fromflowing out of the smelter. An opening 1 is provided for draining thesmelter at the completion of operation.

Three electrodes 8 are introduced through the cover to project below thenormal glass level. Electric current from a 3-phase delta 230 voltsource is conducted through the electrodes and then through the moltenglass 9. Heating is accompl shed by the Joule effect of the moltenglass. The specific resistance of the glass is 0.5 to 2.0 ohms per cubicinch in smelting temperature.

Raw ingredients to be melted are introduced by means of a continuousfeeder I!) through an opening l l in the smelter. The elevation of thefeeder is such that there is always a pile of raw material floating ontop of the bath and above the level of the feeder to minimize dust loss.The raw materials cover the entire bath and melt from the bottom. Theheat loss is thus greatly reduced and the volatile ingredients retainedmore completely.

The power consumption is 250 kw. hr., with frit production of 700lbs./hour. About 357 watts per pound of frit are consumed.

Since this type of smelter operates at 230 line voltage, the currentused is dependent on the depth of the electrodes in the bath. The deeperthe electrodes, the greater is the contact area and the total resistancepath between the electrodes is correspondingly lower. The electrodes maybe raised and lowered to maintain the desired line amperage, either byhand cranking the electrode winches orby push button operation ofreversible motors to operate the winches.

Automatic control also is accomplished by means of current relays whichoperate each electrode motor to maintain desired line amperage.Referring now to Fig. 8, the secondary of the current transformer 30 isconnected in series through in the raw material depth. Furthermore, thegraphite electrodes are slowly eaten away under the bath line so thatcontinuous electrode feed is necessary. Since continual adjustment ofthe electrode depth is required to maintain uniform conditions, theautomatic controls are desirable.

Referring now more especially to Figs. 4 and 5. this smelter has beenprovided with a fining zone l3 which is heated independently of themeltin is supplied by single phase transformers which the ammeter andtwo current relays 32 and 33.

Each relay can be adjusted to make or break contact at any value between1 and 5 amperes. The contacts of relay 32 are connected to close whenthe current falls below its rating. Relay 33 makes contact when thecircuit exceeds its rating. When the contacts are made according to thevalue of the secondary current the reversing contactor 31 is energizedto operate the electrode motor 38 in a direction to either raise orlower the electrodes until the current in the lines and the secondarycircuit has returned to normal.

If a current transformer of 1000 to 5 ratio is used and it is desired tooperate the smelter at 750 amperes on each line, relay 32 will beadjusted to make contact at about 3.5 amperes. The contact will be helduntil the current has reached 3.75 amperes where it will open. Thiscorresponds to 750 amperes in the primary lines. When the current fallsto 700 amperes (3.5 amperes secondary) contact is again made until thecurrent returns to 750 amperes. Likewise relay 33 is adjusted to makecontact when the secondary current reaches 40 amperes (800 amperesprimary). The electrode motor then raises the electrode until thecurrent has dropped to 3.75 amperes.

The current flow is not steady due to the vigorous action around theelectrode and to variation may have variable voltage taps to control thefining power. The resistance of the molten porcelain enamel in thefining zone is much more uniform than in the melting zone where rawmaterial tends to cause bath resistance fluctuations. Therefore,automatic control over the power input to the fining zone is notnecessarily required. The voltage range is to volts. Only 20-25% of thepower input of the melting zone is required in the fining zone sincethis heat is only required to maintain the molten porcelain enamel atthe proper temperature. The fining zone is of such length and volumethat when raw bath ingredients are fed to the melting zone the porcelainenamel discharged at 5 by overflowing will be smelted to the correctdegree.

Total power consumption of this smelter is 500 kw. hr., with fritproduction of 1200 lbs/hour.

About 417 watts per pound of porcelain enamel are consumed.

Openings IS in the melting and fining zone are connected to a stack tocarry off fumes from the smelter. Since the surface of the glass in themelting zone is almost completely covered with raw material there isvery little heat loss in the stack gases.

The smelter is started by filling the melting zone l2 with raw material,with the bridge wall 4 lowered to act as a gate. A piece of graphite islaid in the raw material directly under two electrodes so that whenlowered their pointed ends will touch the graphite. By using a currentlimiting reactor in the line an arc can be placed between each electrodeand the piece of graphite. After a short time a puddle is formed intowhich the electrodes can be lowered, the piece of graphite removed, andheating continued by direct resistance. As the molten portion enlargesthe third electrode is inserted and the bridge wall is raised allowingthe molten porcelain ena'mel to flow into the finding zone.

While the aforementioned vertical electrode smelters are constructed inaccordance with the principle of our invention and are adapted to carryout the process comprising our invention they have certain disadvantagesnamely: relatively high electrode consumption per pound of frit,excessive overheating around the electrodes which causes vigorousboiling of the bath which in certain types of enamels is harmful,uncertain control of power, and excessive wear of the smelter bottom.Therefore in the preferred embodiments of our invention we use a smelteras illustrated in Figs. 6 and '7 which overcomes these previouslyenumerated difficulties. 7

Referring more specifically to Figs. 6 and 7, the smelter hereillustrated makes use of side wall electrodes having an area of contactwith the molten porcelain much larger than that with the use of verticalelectrodes. The resultant lower current density eliminates overheatingadjacent each electrode and produces a more uniform temperaturecondition in the entire bath.

The melting zone 20 and fining zone 2| are separated by a refractorywall 22 having a throat 23 for passage of molten porcelain enamel fromthe former zone to the latter. This solid wall with the submerged throatis more satisfactory than the suspended bridge wall since it has longerlife. A flue opening 28 is located in the roof of the melting chamber20. Two passageways 29 are provided through the wall 22 just below theroof of the fining chamber, for the passage of hot gases. In this mannerthe hot gases flow from the fining chamber into the melting chambercounter currentto the flow of molten'material.

One of the major problems in developing this smelter concerned theelectrodes. If graphite were used it would be necessary to provide ameans of replacement since molten porcelain enamel will erode graphiteeven under the best of conditions. A study of electrode materialsrevealed that pure molybdenum metal was highly resistant to the actionof molten porcelain enamel. Tungsten might also be used but would bemuch more costly. Nickel was found to be useful only with certain typesof poreclain enamels. Although the molybdenum metal withstands thecorrosive action of the molten porcelain enamel it will burn readily ifexposed to air at elevated temperatures. For this reason, the entireelectrode must be kept below the bath level at all times. When thesmelter is drained at the end of a run some enamel will adhere to theface of the electrode thus protecting some portions from oxidation.However, the edges and corners will not remain sufliciently covered. Theuse of an inert gas such as carbon dioxide to flood the smelter whiledraining will prevent serious oxidation. Thus proper care duringdraining will insure long life of molybdenum electrodes.

The rectangular molybdenum plates 24 are located at the sidewalls sothat they are completely immersed in the molten porcelain enamel.Current is conducted to them by water cooled pipes 25 inserted throughthe sidewalls. A layer of chilled porcelain enamel surrounds the watercooled pipes at the point where they are inserted through the wall ofthe smelter, thus preventing the molten enamel from escaping.

The area of these electrodes is larger than that obtainable by usingvertical electrodes as in the previously discussed smelters. Thisreduces the current density surrounding the electrode and eliminateslocalized overheating. The vigorous boiling around the electrodes isthus eliminated and better contact is made between the molten porcelainenamel and the electrode. With the side wall electrodes there is nocontact resistance as in the vertical electrode smelters. Thus the onlyresistance is that of the bath itself which is governed by the spacingbetween the electrodes, bath depth, and width.

The total resistance then is less than in the vertical electrode typewherein the total resistance is the contact resistance plus the bathresistance. Therefore, a lower voltage is required on the largerelectrodes than is used on the vertical electrodes. Since the bathresistance hinges in proportion to the bath temperature it is alsonecessary to have variable voltage control to compensate for temperaturevariation.

This smelter makes use of variable voltage control through saturablecore reactors. A power transformer is used to supply power in thedesired voltage range dictated by the dimensions of the hearth. Thesaturable reactor consists of a series of A. C. windings on a core alongwith D. C. windings. By allowing a very small direct current to flow inthe D. C. windings the A. C. voltage is regulated. If the direct currentis increased the A. C. voltage to the load increases and vice versa.Thus the voltage to the electrodes is varied in a stepless manner in anydesired range. The output voltage may be controlled bythe bathtemperature or by power input, as indicated by a, controlling pyrometer.

Both the melting zone and the fining zone are controlled by separatereactors and transformers. In this way the power and temperature in eachzone can be controlled independently to suit the desired conditions.

The raw material is introduced by two screw feeders 26 so located touniformly distribute the material over the molten bath. The elevation ofthe feeders is such that there is always a pile of 30 raw material aheadof the feeder discharge. The

raw material is pushed into the smelter rather.

than dropped, thus reducing dust loss through the stack. The moltenporcelain enamel is discharged at spout 21 and the rate is governed bythe rate of feed of raw material, although the power input must beregulated according to the rate of feed.

Broadly stated this invention comprises the process and apparatus forsmelting vitreous enamels by passing an electric current through a massof enamel forming constituents and heating such mass to a fluid state byits own internal electrical resistance.

This process has the advantage of being able 0 to heat internallysubstantially uniformly the entire mass. As previously stated the fuelfired smelter causes segregation of the enamel batch due to the surfaceheating and subsequent separating out of some of the ingredients atlower temperatures.

Prior art practices have shown the use of electric smelters for themanufacture of clear glass, however, these prior art electric smeltersar not applicable to the manufacture of porcelain en- 55 amel. In themanufacture of clear glass transparency is of the utmost importance andit is therefore most essential that the molten materials from which theglass is made is absolutely free of undissolved or suspended materialknown to those versed in the art as seeds.

Therefore in the fining of clear glass all smelters must be soconstructed that the discharged molten material is 100% fined.

The design and function of our electric smelter for porcelain enamelmust be different from that used in the manufacture of clear glass.Porcelain enamel may be generally defined as a composition whichincludes usually a sodium-borosilicate glass matrix in which are held inuniformly distributed suspension the opacifying compounds which areusually undissolved. The function of the smelter for the manufacture ofporcelain enamel is therefore, not to separate out the crystalline orundissolved components such as the opacifying agents but to insure thatthe same will be evenly distributed throughout the entire bath so thatthe resultant product may have uniform characteristics such as opacity,etc.

Furthermore in the manufacture of clear glass, the glass batch containslittle or no fluoride compounds. As is well known the fluoride compoundsare extremely corrosive and cause rapid destruction of electrodes andrefractories unless the smelter is constructed so as to minimize thisdestruction.

It can now be readily seen that an electric smelter for the manufactureof porcelain enamel must be able to first overcome the problem ofsegregation during the melting process, secondly the smelter mustdischarge a molten mass that is not 100% flned, and thirdly the smeltermust be constructed so as to minimize the destructive influence offluorides.

Electric smelters usually generate higher temperatures than fuel firedsmelters and one would naturally expect that the result of usin anelectric smelter would be to increase fluorine loss. However, we haveprovided a method and apparatus in which the opposite is true.

The use of our electric smelter for producing vitreous enamels hasgreatly minimized the loss of fluorides during the smelting process. Dueto the fluorine retention during electric smelting it is possible toproduce a vitreous enamel from a raw batch containing 4% less fluorinethan ordinarily would be used in producing the same enamel on a fuelflred smelter.

The fluorine retention during electric smelting applys to both classesof enamels known to the trade as Cover Coats and Ground Coats.

So that the foregoing statements are more clearly understood thefollowing examples are iven:

Example I The following formula is atypical zirconium bearing whitecover coat enamel The above raw batch when smelted on a fuel firedsmelter produced a frit having an average of 7.77% fluorine. Thisconstitutes a loss of 4.61% fluorine during the smelting process.

The same formulation as above when smelted in an electric smelterproduced a frit containing 10.45% flourine. This constitutes a loss ofonly 1.93% fluorine during the smelting process.

From the foregoing Example I it now becomes apparent that the electricsmelter produces a vitreous enamel with less loss of fluorine.

It will be observed that when employing the process and apparatuscomprising my invention, the raw materials from which the finishedporcelain enamel is to be produced, are fed into the melting chamber andare quickly and efliciently reduced to a molten state for discharge tothe fining chamber. The raw materials are heated internallysubstantially uniformly and the possibility of overburning theingredients has been brought to an absolute minimum.

Our invention has the further advantage of keeping the fluorine loss atan absolute minimum thus making it possible to produce porcelain enamelfrom a raw batch which contains less fluorine than heretofore has beenused.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in the following claim, or the equivalent of such, beemployed.

We therefore particularly point out and distinctly claim as ourinvention:

A smelter for the continuous production of porcelain enamel comprisingthe combination of a hearth comprising melting and fining zones, aconfining structure comprising upstanding walls and a roof, arrangedabout and over said hearth and forming therewith substantially closedadjacent melting and fining chambers, said melting chamber provided witha charge opening for raw material and a flue opening for discharging hotgases and said fining chamber provided with a discharge opening formelted material, the material supporting surface of said hearth beingsubstantially a straight line inclined towards said discharge opening,separate passages respectively for hot gases and melted materialconnecting said chambers, said passage for hot gases disposedsubstantially adjacent the roof of said fining chamber wherein the hotgases flow countercurrent to the flow of the molten material and saidpassage for melted material being restricted and submerged below thenormal bath level, four horizontal electrodes disposed below the normalbath level and arranged in pairs opposite each other in the side wallsof said melting zone and two horizontal electrodes disposed below thenormal bath level and arranged opposite each other in the side walls ofsaid fining zone, said electrodes being of the same polarity on one sideand of opposite polarity on the opposite side, a power circuitsuflicient to supply an electric current through the electrodes in saidmelting chamber to effect a heating current flow through a mass ofporcelain enamel forming materials, and a separate power circuit, insaid fining chamber sufficient to supply an electric current to maintainthe said melted material in fining chamber in a molten state.

ROBERT E. SKINNER. GLENN H. MCINTYRE.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Number Name Date 1,267,317 Erskine May 21, 19181,552,555 Grauel Sept. 8, 1925 1,610,377 Hitner Dec. 14, 1926 1,799,980Hartford Apr. 7, 1931 1,820,248 Raeder Aug. 25, 1931 1,827,471 HitnerOct. 13, 1931 1,880,541 Wadman Oct. 4, 1932 1,897,973 Wadman Feb. 14,1933 1,905,534 Wadman Apr. 25, 1933 1,928,289 Henry et a1. Sept. 26,1933 1,944,855 Wadman Jan. 23, 1934 2,089,690 Cornelius Aug. 10, 19372,122,469 Hitner July 5, 1938 2,159,361 Atkinson et al. May 23, 19392,283,188 Cornelius May 19, 1942 2,314,956 Slayter et al Mar. 30, 19432,413,037 De Voe Dec. 24, 1946 2,471,531 McIntyre et a1. May 31, 1949

